There are many different versions of the so called positive displacement pumps or motors or flowmeters, which are employed in moving fluids or harnessing power from moving fluids or measuring the flow rate of fluids. These positive displacement devices employed as a pump or motor or flowmeter operate on the principles that the flow of fluid and the rotating motion of the rotor included in the positive displacement device are positively or semi-positively coupled to one another, whereby the rotating motion of the rotor positively moves the fluid or the motion of the fluid positively rotates the rotor. With few exceptions, most of the existing versions of the positive displacement pumps, motors and flowmeters suffer from at least one of the two short-comings of the following description: Firstly, the most serious problem with the existing positive displacement apparatus is the high frictional resistance between the rotor and rotor housing of the positive displacement device, which frictional resistance is primarily responsible for the low efficiency of the pumps and motors operating on the principles of positive displacement as well as for the inability of the flowmeters of positive displacement types to measure very low flow rates of fluids. Secondly, existing positive displacement devices whether used as a pump or motor or flowmeter have ungainly large bulk in comparison with the actual capacity of fluid flow handled thereby. In other words, the overwhelmingly large portion of the wetted volume in the existing positive displacement devices are occupied by the rotor assembly, which leaves only a small portion of the wetted volume to be occupied by the fluid. This feature of the existing positive displacement devices is highly undesirable in terms of functional aspect as well as the economics of operation as the unnecessarily bulky and heavy devices are expensive to manufacture and inefficient in operation. The present invention teaches a positive displacement device that has a highly efficient construction and a very low friction between the rotor and the rotor housing of the device, which positive displacement device may be used as a pump or motor or flowmeter.